Epsilon caprolactam is a valuable intermediate which is useful, for example, in the production of nylon 6. Certain processes used to produce epsilon caprolactam generate isomer byproducts, e.g., 4-ethyl-2-pyrrolidinone, 5-methyl-2-piperdinone, 3-ethyl-2-pyrrolidinone and 3-methyl-2-piperdinone, some of which exhibit less than a 1.degree. C. difference in vapor pressure at 10 mm Hg (absolute) compared with caprolactam. Conventional separation techniques such as distillation are not effective for separating such mixtures. Accordingly, it would be desirable to separate epsilon caprolactam from its isomer byproducts to give high purity epsilon caprolactam.